Intake valves for different automobiles
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Your Inlet valves (intake valves) worn out? » Read more about common malfunctions • Repair • Replacement manual
Supplying the engine with air: the intake valve
Before the era of the injection pump, engines had carburettors (Am. Eng.: carburettor or gasifier). Pistons moving downward generated suction pressure, which in its turn caused the fuel/air mix to flow into the combustion chamber via the intake valve. Nowadays, intake valves only let in air. The fuel is sprayed into the combustion chamber by fuel injectors.
Intake valve: function and location
The injection valve is located in the cylinder head under the camshaft. An average engine has one intake and one outlet valve per cylinder. Sportier engines have two of each per cylinder. The first cycle in a four-stroke engine is suction. This is the moment when the intake valve opens. Air flows into the combustion chamber and mixes with fuel by the injector. When the piston has reached the bottom dead center, the intake valve closes, sealing off the combustion chamber. The piston rises again, compressing the air/fuel mix until it is sufficiently compressed to cause a powerful explosion. The explosion power generates the operational pressure by which the piston is pushed down again. The flywheel or a neighbouring piston supplies the energy to allow the piston to rise again. Now the outlet valve opens and the burnt exhaust escapes. Immediately, the outlet valve closes and the intake valve opens. The cycle starts all over again.
Intake valve defects
An intake valve failure has three possible causes:
- Mechanical damage by pistons colliding with the valve
- thermal stress through overheating
- fracture of the valve disc.
A mechanical damage caused by pistons occurs when the valve control fails. The rising piston slams against the valve when it is still hanging in the combustion chamber, resulting in a bent valve stem immobilising the valve. An engine running too lean generates thermal stress, causing the valve to lose its fitting shape. Lastly, the valve can fracture as a result of alien elements in the intake manifold or due to overaging. Adequate compression development is no longer possible and the car's performance is impaired.
Repair of the intake valve
An intake valve costs only ca. 15 EUR (± £13.25). Its installation is a lot of work and requires plenty of expertise. If this knowledge is not present, a repair attempt can cause excessive damage. Therefore it is absolutely necessary to acquaint yourself with this repair before unscrewing the first bolt. The separate steps are roughly the following:
- removing the valve cover
- disassembly of the cylinder head
- thorough cleaning of the cylinder head
- removing the valve
- if necessary: milling the valve seat
- replacing all valve stem gaskets
- installing and grinding in of the valve
- replacement of the hydraulic tappets
- renewal of the valve control
- replacement of the bearing cups for the camshaft
- replacing the cylinder head gasket
It is better to replace all valves instead of just one. The result is cylinder head as good as new. If valve seats require milling, it is a good idea to install hardened valve seats. This will extend the life span of the new valves considerably. This repair requires several special tools. For the inexperienced DIY'er it might be a better idea to just remove the cylinder head and have it reconditioned by a garage. This guarantees a correct repair. A cylinder head overhaul costs ca. 400 – 600 EUR (± £350 - £530) for an average car. The service takes about a week. If you are in a hurry, it is better to get yourself a completely overhauled cylinder head by exchange. As a rule, the defective component has to be sent to the service provider.
Costs for an intake valve
The price for a separate intake valve starts at ca. 15 EUR (± £13.25) apiece. Correspondingly, a complete valve set costs ca. 120 – 240 EUR (± £106 - £212), depending on whether for a 8V or a 16V engine. The valve stem gaskets must be replaced in any case, which will set you back another 50 EUR (± £45). For the cylinder head plus accessories you pay another 150 EUR (± £135), more or less. All in all the valve replacement is an expensive business, which must be planned carefully. With a bit of exercise and the necessary special equipment, a DIY'er can attain a valuable proficiency.